A security that is not registered with the SEC and thus, cannot be sold in the marketplace. The issues are sold under an investment letter in which the purchaser states the purchase is for investment purposes and not for resale. The certificates have a restrictive legend that indicates they are not registered. Because the investment letter is essential to the security's issuance, this type of security is called either "letter security", "letter stock", or "letter bond". See: Registered Security; Unregistered Stock